1. Field of the Invention
The present invention relates in general to automatic transmissions for motor vehicles, and more particularly to multispeed automatic transmissions such as one having five or over five speeds. More specifically, the present invention is concerned with a control system of such multispeed automatic transmissions.
2. Description of the Related Art
With increase of speeds needed by an automatic transmission, the number of frictional elements, such as clutches and brakes installed in the transmission increases accordingly. Furthermore, with increase of the speeds, the speed change lines of a shift map become denser, and thus, in such multispeed automatic transmission, the actual speed change takes place much shortly or easily even in a small change in operation condition of the vehicle, that is, for example, a small change in throttle opening of the engine. Actually, as the distance of speed change lines becomes smaller, so-called two skip shift (for example, shift from 6th speed to 3rd speed) or three skip shift (for example, shift from 6th speed to 2nd speed) tends to occur frequently.
In current automatic transmissions, there is a type wherein a gate type speed change lever is installed for allowing the driver to positively select his or her desired speed disregarding the automatic shift of the transmission. Furthermore, for the same purpose, there is another type wherein a speed change paddle or button is arranged on a steering wheel. In the automatic transmissions of such types, the two or three skip shift takes place much frequently due to the nature of such positive shift by the driver.
Usually, in the automatic transmissions of such types, there is employed a speed control unit that exhibits a satisfied speed control in case of one skip shift in downshift direction (for example, shift from 4th speed to 2nd speed). Actually, in such one skip downshift, the speed change is carried out based on a given speed change control program memorized in a memory section of the control unit.
Thus, in case of the above-mentioned multispeed automatic transmissions, it is necessary to provide the control unit with a speed change control program that is suitable for various speed change patterns including one, two and three skip shifts. However, in this case, the control program becomes quite complicated and thus the amount of data to be handled by the control unit becomes enormous thereby causing usage of a large capacity storage in the control unit. Among the shifts including the skip shifts, a shift that is free of a speed change control program will be referred to as sequential shift in the following.
For eliminating the drawbacks mentioned hereinabove, various measures have been hitherto proposed in such type automatic transmissions, which are those disclosed in Japanese Laid-open Patent Application (Tokkaisho) 62-46055 and International Laid-open Patent Application 95/12774.